There are various applications where it is desirable to maintain a stable output wavelength from a laser source. For example, a major requirement for lasers installed in gyroscope systems is to be able to generate a consistent wavelength over a vast temperature range (e.g., −60° C. to +90° C.). Optical amplifiers also require a stable laser source, in this case for providing a stable “pump” input to the amplifier. In many embodiments, a pump input at a wavelength of 980 nm is used to provide amplification within an erbium-doped optical fiber. If the output wavelength of the pump starts to drift by even a few nanometers (which may easily happen in the presence of changes in ambient temperature and/or input drive current), the gain and output power from the doped fiber is reduced and the efficiency of the optical amplifier is compromised.
Conventional approaches of “wavelength locking” a laser source typically utilize a fiber Bragg grating (FBG) inscribed within the optical output fiber coupled to the packaged laser source. The FBG functions as a highly-selective wavelength filter, further ensuring that only the desired 980 nm wavelength beam continues to propagate along the output path. As will be explained below, this FBG needs to be positioned well beyond the output from the laser source (on the order of about 1-2 meters), so as to properly interact with the front facet of the laser diode to create a reflective, external cavity that functions to further stabilize the wavelength of the laser output light.
Well-known processes for creating an FBG in an optical fiber require significant handling of the fiber itself. For example, the portion of the fiber within which the grating is to be located must be processed (“stripped”) to remove outer jacket and coating layers, exposing the bare fiber where the grating is formed. Next, the stripped portion of the fiber is typically exposed to a UV source that illuminates the fiber with interfering beams in a manner that modifies the refractive index profile of the core region of the fiber as a function of the beam interference, forming the grating. Once the grating is formed, new layers of jacket and coating material need to be re-applied along the stripped area of the fiber to protect the grating.
Throughout these and other processes, the fiber exiting the laser diode package is subjected to an extensive amount of handling, which may lead to further degrading the long-term reliability/strength of the fiber (especially in situations where the fiber needs to be tightly coiled to comply with packaging requirements). The degradation in the long-term reliability is due, at least in part, to defects introduced in the fiber during the process of creating the grating structure.
Conventional methods of accommodating the presence of these defects include the use of special packaging constraints that maintain the portion of the fiber containing the grating in a straight line so that any bend-induced defects are avoided. Alternatively, special packaging can be created that maintains a large bend radius within the section of fiber containing the grating. Given the lengths of fiber associated with these systems (typically, at least one meter of fiber; in most cases in the range of 1-2 m) the demands on “special packaging” may be greater than economically feasible for most applications. Moreover, there is an increasing demand to comply with “small form factor” packaging requirements for all optical components, where these requirements do not easily accommodate the use of relatively long lengths of fiber or large radius fiber coils, as common in the prior art, to provide output wavelength stability from laser sources.